(a) Technical Field
The present invention relates to a power transmission device for a hybrid vehicle. More particularly, the present invention relates to a power transmission device for a hybrid vehicle, which implements a series-parallel type power transmission path capable of varying an engine output for each driving mode, preferably using a Ravigneaux gear train and two or more clutches, thereby improving the fuel efficiency of the hybrid vehicle.
(b) Description of the Related Art
In general, a power transmission device for a hybrid vehicle can be configured in various combinations, using an engine and a motor, where the power transmission device typically includes a serial power train, a parallel power train, a series-parallel power train, and the like. In particular, the serial power train allows driving of the vehicle to be performed through driving of the motor in a state in which the engine serves only as a power generator, and the parallel power train allows driving of the vehicle to be performed by combining power of the engine with power of the motor. The serial-parallel power train allows driving of the vehicle to be performed using the engine and the motor together. In this case, the power of the motor or engine is distributed according to driving conditions.
FIG. 8 is a power transmission system diagram showing one configuration example of a conventional series-parallel power train for a hybrid vehicle. The series-parallel power train includes an engine 1, a first motor (MG1) for generation, a second motor MG2 for driving, a first gear 2-1 connected to the second motor MG2, a second gear 2-2 connected to the engine 1 by a clutch 6, a third gear 2-3 simultaneously engaged with the first gear 2-1 and the second gear 2-2, and an output gear 3 outputting power to a driving wheel 7 while keeping the same axle with the third gear 2-3. Accordingly, a first HEV driving mode and a second HEV driving mode are implemented as well as an EV driving mode.
EV Driving Mode
The EV driving mode is a driving mode in which the initial low-speed driving of the vehicle is performed through only driving of the second motor MG2. In the EV driving mode, the driving of the vehicle is performed by driving only the second motor MG2 based on power of the battery 4 in a state in which the engine 1 and the first motor MG1 are stopped. The driving force of the second motor MG2 is transmitted to the output gear 3 through the first gear 2-1 and the third gear 2-3 and then transmitted to the driving wheel 7, thereby performing the driving of the vehicle.
First HEV Driving Mode
Like the EV driving mode, the first HEV driving mode is a driving mode in which the initial low-speed driving of the vehicle is performed through only driving of the second motor MG2. Particularly, the first HEV driving mode includes transmitting power of the engine 1 to a driving gear 8 of the first motor MG1 through an input gear 5 at the same time when the engine 1 is started, generating power by driving the first motor MG1 using power of the engine 1, and charging electric power generated by the first motor MG1 in the battery 4.
Second HEV Driving Mode
The second HEV driving mode is a driving mode in which the high-speed driving of the vehicle is performed using power of the engine 1 as auxiliary power in addition to power of the second motor MG2. The second HEV driving mode includes transmitting power of the engine 1 to the second gear 2-2 through the clutch and simultaneously transmitting the power to the third gear 2-2 engaged with the second gear 2-2, and transmitting the power of the engine 1 to the driving wheel 7 through the output gear 3 connected to the third gear 2-3 on the same axle and a reduction gear.
However, the conventional series-parallel power transmission device described above has at least the following disadvantage.
The input/output path through which the power of the engine is transmitted to a final output gear is limited as one path in the engagement of an engine fixing gear stage, i.e., the clutch. Therefore, there is little possible improvement of fuel efficiency as compared to a conventional vehicle.
In other words, the gear train disposed on the power transmission path through which the output shaft of the engine is connected to the final output gear, i.e., the input/output path of engine power, is combined into a structure in which the third gear and the output gear are simply connected in addition to the spur gear-type second gear, and therefore, the engine power is not appropriately distributed to the driving wheel according to the speed of the vehicle. Accordingly, it is difficult to attain improvements of fuel efficiency.